Spokeswoman Bernadette Meehan said the US is also extremely concerned" that thousands of Palestinians aren't safe in UN-designated shelters in Gaza, even though Israel's military has told them to evacuate their homes.

Ms Meehan said the US also condemned those responsible for hiding weapons in UN facilities in Gaza. She says the escalation shows the need for a cease-fire as soon as possible.

Video: The latest from Gaza

Kim Sengupta, The Independent's Defence and Diplomatic Correspondent, tweeted on Wednesday that he had recently visited the school in Jabaliya, where people were "convinced" the building would soon be hit.

#Gaza Had met people at Jabaliya school ten days ago. " Told you that you'll be back" said Mohammed Abu Jarad today.

The UNRWA said today it is at "breaking point" with over 200,000 displaced Palestinians seeking refuge in 85 of their shelters following calls by Israel for civilians to evacuate whole neighbourhoods before military operations.

The death toll has now reached almost 1,300 Palestinians and 56 Israelis since 8 July.